Vacuum extrusion of high quality, low density polystyrene foam board sold under the well known color PINK.RTM. and FOAMULAR.RTM. trademarks by Owens Corning of Toledo, Ohio, USA, has been accomplished in inclined barometric leg vacuum extrusion lines. In such systems, the vacuum chamber is somewhat inclined. The die is positioned at the upper end along with shaping or calibration equipment. At the lower end, the chamber is closed by a hood extension and immersed in a pond of water. The pond seals the lower end of the chamber and provides an immersion cooling bath for the extrudate as it leaves the vacuum chamber. The buoyant extrudate may be supported beneath a continuously moving belt which moves through the pond through a large radius of curvature. When the extrudate surfaces to atmosphere, it is cut and processed further. Such installations are costly and present many operating problems, particularly since the upper end of the chamber may be a number of meters above and a substantial distance from the lower end. Anything dropped at the upper end of the inclined chamber where all of the relatively complex shaping and calibrating equipment is located may literally have to be fished out of the lower pond many meters away.
In U.S. Pat. No. 4,783,291, a horizontal vacuum chamber system is employed which seals the exit end of the chamber with a water baffle seal. The extrudate exits through an underwater orifice which connects the vacuum section and atmospheric section of an immersion cooling pond. The extrudate is conveyed through the orifice by a curved belt conveyor, and the top of the orifice has a movable shutter which restricts the orifice in response to vacuum level. The shutter acts as a gross flow control valve for water moving from the lower level atmospheric section of the pond to the higher level vacuum section of the pond. During vacuum operation, the level of the pond inside the chamber is maintained by circulating excess water back to the atmospheric section.
For sizable or complex extrudates, relatively complex power driven and adjustable equipment is required downstream of the die. For a fan shape die, where the die lips are curved, the equipment may literally surround the die. Typical of such equipment is an apparatus known as a "slinky" which includes upper and lower sets or assemblies of power driven disks which are mounted for rotation on arcuate or curved axles which extend at different radii from essentially the same center as the curvature of the fan shape of the die lips. All of such shaping and calibration equipment is complex and requires access and servicing, particularly during start up.
A foaming, moving, hot extrudate under vacuum is an amorphous object and does not become substantially fixed until it passes through the cooling immersion pond of the water baffle seal to atmosphere. If the shaping or calibration machinery is not functioning properly, the amorphous extrudate may become deviant, expanding or diverting from the machine line. When this happens, more often during startup, the problem needs to be corrected promptly to avoid shutting down the line. If the line is shut down for any significant length of time, equipment may have to be removed and replaced or throughly cleaned before the line can be restarted. Downtime versus operating time, and rate is the economic measure of any production facility. It is accordingly important that the equipment be quickly accessible, and that the extrudate be drawn through the system without being pushed or shoved with inconsistent or excessive force. It is also important that the underwater exit orifice closely match the size of the extrudate which may vary in width and thickness. Too large an opening creates inefficiencies, while too small an opening can create hangups, deviations, pull-aparts, and other problems.
In the extrusion production of foam boards, such as the noted insulation boards, the size and thickness may be substantial, such as 10 to 12 centimeters (3.94 inches to 4.72 inches) in thickness and up to a meter or more wide. Such board may have a cross-sectional area of in excess of about 1000 cm.sup.2 (155 in.sup.2). To make such board in economic quantities, such as more than 450 kg/hour (1000.0 lb/hour) to about 1360 kg/hour (3000.0 lb/hour) or more, the system must have substantial throughput and achieve a uniformity of the melt. To achieve proper uniform cell size and structure for low density, large size product such as those having a cross sectional area of at least 80 cm.sup.2 (12.4 in.sup.2) and preferably from about 200 cm.sup.2 (31 in.sup.2) to about 1000 cm.sup.2 (155 in.sup.2) or more, the proper uniform melt must be formed.
The melt is formed from pellets and reclaim scrap and other additives by the extruder under heat and high pressure. The other additives may include fire retardant and UV inhibitors, for example. A blowing agent is also added which does not expand in the melt under pressure, but does so as the melt exits the die into the vacuum chamber. The vacuum increases the pressure difference, promotes the expansion and enables the production of low density foam.
As is known, the melt has to achieve certain elevated temperatures for thorough mixing and formation of the melt, but to achieve uniform quality foam product, particularly in a low density vacuum foam system, a critical uniform viscosity range must be achieved. The particular viscosity range is dependent on product size and density. A higher viscosity is required for larger size product. If the product is not viscose enough or too fluid, the cells will rupture or collapse during foaming. If the melt is too viscous, homogeneous cell grown is difficult to impossible. Although cells may collapse in atmospheric systems, in a vacuum foam system, problems such as cell collapse or less than prime quality product may be more pronounced. A vacuum foam system is different from normal atomospheric foam systems. Not only is there an increased pressure drop at the die lips, but the reversion to atmospheric pressure, especially when emerging from an immersion seal, can result in cell collapse or non-uniformity actually distorting or shrinking the product, resulting in irregularities or density gradients, and less than quality product. In vacuum foaming, not only must the proper viscosity be achieved, it must be maintained uniform throughout the melt. Viscosity is controlled in part by controlling the temperature of the melt.
The problem with many heat exchangers employed for such purposes is several fold. One set of problems is complexity and cost. Another set is effectiveness and efficiency. To move the polymer melt through elbows or right angle turns at high pressure and temperature, or through divergent flow paths is energy inefficient and raises the costs involved. Moreover, niches or potential dead space should be avoided or minimized. These do not contribute to homogeneity of the melt and require more frequent cleaning and downtime for such purposes. Such dead space is simply inefficient. A complex form of heat exchanger is shown, for example, in U.S. Pat. No. 4,423,767.
The flow path of the melt should be as close to or aligned with the machine axis as possible, and the heat exchanger should be as compact as possible. Any excess increase in dimension between the extruders and the die can be self defeating, since any thermal or viscosity homogeneity achieved by the heat exchanger may be lost if the melt has to travel too far. This is further complicated if the die is inside a vacuum chamber to achieve a good low density foam, and if adjustments or thermal expansion or other minor movements need to be accommodated.
While static mixers have been employed to attempt to achieve homogeneity of melts, they do not, nor have the capacity or efficiency necessary for the large throughputs noted above, and the production of quality foam products subject to the pressure changes of vacuum extrusion.
To achieve both extrusion throughput rates and product quality, it is important to have a mixer which can also precisely control the temperature and thus the viscosity of the melt and maintain the thermal homogeneity to the die. Only in this manner can the benefits of high quality low density foam formed under vacuum be achieved, reducing density gradients in the foam, which gradients may result in or from cell or board collapse particularly as the board moves from the vacuum chamber to the pressure of atmosphere. To achieve this improved product quality for a range of products which may vary in cross section (from relatively thin to thick) and vary in density, the heat exchanger must be able to control the melt temperature very precisely, and maintain homogeneity of temperature all at varying throughputs, and most difficultly at high throughputs for large extrudates.